A railway vehicle typically has axles mounted in pairs to form a truck at each end of the railway vehicle. The trucks typically pivot about a central axis of the truck midway between the two axles. When the vehicle traverses a curve, the vehicle chords across the curve from truck axis to truck axis.
This chording across the curve creates an overhang at the center of the truck over the rails, and the limit on the allowable overhang limits the permitted truck spacing. Also, the constraints of the usually rigid load bearing structure of the truck prevents the axles from moving into radial alignment with the curve, and it has previously been found difficult to overcome these constraints. If the wheels are not radially aligned on a curve, then the wheels are subject to increased instability and wear.